Though i only use when I'm cruising, if I'm going up a hill or speeding up to merge into I turn it off. I recall A/C reduces HP?
How much? like a lawn motor 2.5 hp?

Last question is regarding my radio system. It doesn't really affect the alternator right? I only have aftermarket amp 250WATT and a deck. I'm planning on using an 120WATT adapter to charge my laptop on a long trip. That won't affect my gas mileage right? I had a friend whom wine about how it would take power away from the engine...

Theoretically that doesn't make sense to me. An alternator (properly our generate about 80AMPS) and it's charging the battery and when the battery is full, the power is wasted. Take power away from i.e. 120 watt wouldn't really affect charge or gas usage at all. Does this make sense, i just need a confirmation.